Code Division Multiple Access (CDMA) modulation is a multi-user access transmission scheme in which signals from different users overlap both in frequency and in time. This is in contrast with Frequency Division Multiple Access (FDMA) in which user signals overlap in time, but are assigned unique frequencies, and Time Division Multiple Access (TDMA) in which user signals overlap in frequency, but are assigned unique time slots. CDMA signaling is frequently used in cellular communication systems between a base station within a cell and a plurality of access units, e.g., wireless handsets, in the possession of users within the cell. The CDMA transmitted signal for each user that broadcasts from the user's access unit is spread over a wide bandwidth, which is greater than the initial user information bandwidth. Each user's signal is spread by a different spreading code to create a wideband spread. All of the spread wideband signals transmitted by the different users are received at the base station and form a composite received signal. The receiver at the base station distinguishes different users by using a local copy (or local reference) of the spreading code, which is available to both the access units and the base station in the CDMA system. Such a process is called channelization.
In an exemplary CDMA system according to the IS-95 standard, channels are defined for a reverse link, i.e., when an access unit is transmitting to a base station in the system, using a code called a pseudorandom noise (PN) code. The receiver at the base station detects the desired signal from a particular user out of the composite signal by correlating the composite signal with the original PN code. All other signals having codes that do not match the code for the desired user code are rejected by the correlator.
An exemplary CDMA reverse link includes a plurality of channels, e.g., access and traffic channels (or even more channel types depending on the design of the CDMA system). The traffic channel is used to transmit user data and voice, as well as signaling messages. The access channel is used on the reverse link to communicate control information to the base station. For example, when the access unit does not have a traffic channel assigned, the access channel is used to make call originations and to respond to pages and orders. The traffic channels are principally used to communicate voice or data payload information but are also used for other functions.